Page:The story of the comets.djvu/27

I. original appearance, but in the reverse order. To this it may be added that a comet discovered in the Northern Hemisphere usually passes into the Southern Hemisphere after it has made its nearest approach to the Sun, and disappears in that hemisphere. Conversely, a comet discovered in the Southern Hemisphere generally comes North, and disappears in the Northern Hemisphere, but exceptions to this rule are not uncommon.

In size and brilliancy comets exhibit great diversity. It sometimes, but not very often, happens that one appears which is so bright as to be visible when the Sun has not yet sunk below the horizon; but the majority are invisible to the naked eye, and need either a little, or a great deal of, optical assistance. All these latter are "telescopic comets". The appearance of the same comet at different periods of its visibility varies so much that we can never certainly identify a given comet with any other by any mere physical peculiarity of size, shape, or brightness. Identification only becomes possible when its "elements" have been calculated and compared with those of some other comet previously observed. It is now known that "the same comet may, at successive returns to our system, sometimes appear tailed, and sometimes without a tail, according to its position with respect to the Earth and the Sun; and there is reason to believe that comets in general, for some unknown cause, decrease in splendour in each successive revolution". Halley's Comet, which we are all expecting in 1910 or sooner, has been thought to have diminished in brilliancy during the many centuries that have elapsed since it was first recognized, judging by a comparison of the descriptions given of it; but doubts have been cast on this supposition by Holetschek, who concludes that for a thousand years from 837 A.D. to 1835 its magnitude has remained fairly constant, between the 3rd and 4th star magnitudes; whilst between 1456 and 1835 there was no great variation in the length of its tail.

Holetschek has carried out some investigations as to the magnitude and brilliancy of comets and their tails from the